Cooling issues associated with the design of modern computer and communications equipment include the dissipation of high-power levels from temperature sensitive electronic devices. This requires system designers to pay special attention to protecting against temperature hazards while obtaining optimal noise performance.
Conventional cooling tasks in computerized systems commonly include monitoring and controlling the temperature of various temperature-sensitive devices and zones. Speed controlled fans are used for the actual cooling task. Commonly, each monitored zone or device may have a different operating temperature range/limit, which requires that each zone or device be controlled separately to provide the required operating temperature ranges.
Fan speed is controlled commonly through modification of the duty cycle (“DC”) of a pulse-width modulated (“PWM”) signal (hereinafter, broadly defined as a “control signal”). For purposes of illustration, it is assumed that the control signal is “off” when an associated fan is stopped (i.e., providing no cooling function and generating no noise), and is “on” when the fan is rotating to provide cooling power and generating noise (when the fan is at maximum power, the fan is rotating at “full” speed to provide maximum cooling power and generating maximum noise).
Turning initially to FIG. 1, illustrated is a temperature control graph of an exemplary PRIOR ART fan-cooling temperature control scheme. This scheme is commonly known as a linear control scheme. Conventional exemplary linear control schemes (e.g., NSC LM85, SMsC EMC6D100/EMC6D101, ADI ADM1027) may suitably consist of:                a Minimal Temperature limit (“Tmin”) that defines a low temperature limit for activating cooling means in minimal cooling power (“DCmin”),        a Maximal Temperature limit (“Tmax”) that defines a high temperature limit for activating cooling means in maximum cooling power (“DCmax”), and        a Proportional Range in which the DC of the control signal is linearly changed in proportions with the changes of the measured temperature.Under this scenario, when the fan is activated, the fan control performs a “spin-up cycle” in which the control signal may be provided with maximum DC to overcome fan inertia. Additionally, a hysteresis range (“H1”) may be defined to prevent sequential spin-ups that can occur due to temperature fluctuations near Tmin.        
A common problem with linear control schemes occurs from various disadvantages due primarily to the nonlinear nature of the cooling function and control components. For instance, fan speed and DC of the control signal may be represented in quadratic relation, V≅K×√{square root over (PWM DC)}. Such schemes cause abrupt PWM DC changes of the control signal that result in noisy fan speed oscillations.
The efficient cooling function shown in FIG. 1 generates minimal acoustic noise, however, it is nonlinear, and therefore the linear control schemes of the PRIOR ART are not efficient, commonly suffering from significant acoustic noise. Further, additional conditions in controlled cooling schemes may arise due to uncertain factors related to the physical conditions, such as airflow, packaging, available space or the like, in each zone and in the system packaging. It often difficult to approximate an efficient cooling function as a result thereof, particularly since these conditions may vary from one system to the next.
In one example, a modified linear control scheme is implemented that improves temperature control efficiency by dynamically adapting the value of Tmin. By dynamically adapting Tmin as a function of the measured temperature, the slope of the proportional range may continuously be adapted to obtain more efficient cooling, however, this solution commonly requires complex algorithms including different updating cycles to update the Tmin limit value. It is therefore difficult to stabilize the temperature control loop in a specific system as it involves controlling the repetition rate of the two kinds of updating cycles.
More recently, efficiency problems involved in fan speed control schemes were minimized using nonlinear control schemes that are closer to the efficient cooling function of the system. In one example, the fan speed may be controlled by:                gradually increasing the DC of the control signal whenever the measured temperature is above a predefined high limit,        gradually decreasing it whenever the measured temperature is below a predefined low limit, and        keeping the control signal constant/unchanging whenever the measured temperature is within said limits.This control scheme minimizes some acoustic noise by preventing noisy fan oscillations outside limits, however cooling efficiency is not improved since no adjustments of the control signal are performed within the operating range defined by temperature limits.        
An alternate nonlinear control scheme utilizes a multi-step look-up table to program a non-linear transfer function for fan control. This solution provides improved efficiency and reduction in the acoustic noise by allowing rough approximation of a desired nonlinear transfer function. Nonetheless, performing control on the basis of a multi-step look-up table is costly in terms of memory resources and, if the number of steps is low, the scheme may cause abrupt changes of the control signal that result in increased acoustic noise.
In summary, all of the above-described methods have provided solutions to the fan controlled cooling problems that are less than optimal relative to the need in the art. An important goal in cooling system optimization is to generate minimal noise (by setting the control signal at the minimal effective DC) for a given power dissipated by the system. A need therefore exists in the art for efficiently cooling various devices in the system individually or in zones utilizing a nonlinear control scheme. A further need exists in the art for reducing the acoustic noise of fan cooling systems by utilizing a nonlinear cooling control scheme. A yet further need exists in the art for an improved temperature control scheme that allows accurate approximation of desired nonlinear control functions. A still further need exists in the art for an improved temperature control scheme that minimizes the stability problems of the temperature control loop by using a control process that is based a single-loop.